The mechanism by which the apical ectodermal ridge (AER) directs the outgrowth and formation of distal limb structures by limb mesodermal cells, and the related problem of the control of limb cartilage differentiation will be investigated. Studies have shown that the AER exerts a negative effect on the differentiation of the mesenchymal cells directly subjacent to it, and that when the cells in the central core of the limb leave the AER's influence they undergo a cellular condensation process during which their chondrogenic differentiation is triggered. We will test the hypothesis that prostaglandins (PGS) produced during condensation regulate limb cartilage differentiation by locally acting on adjacent mesenchymal cells, and, thereby, elevating their cyclic AMP levels. The possible involvement of Ca++ in limb cartilage differentiation will be probed, as will the relationship between Ca2+, PGs, and cyclic AMP in the process. The nature of molecules produced by the AER compared to limb and non-limb ectodermal tissues that do not support limb outgrowth and to older EARs that have lost their influence on limb mesodermal cells will be investigated. Our studies on hyaluronate (HA) synthesis and degradation in limb development will be continued by determining if HA turnover and increased hyaluronidase activity occur as cells leave the AER's influence and initiate the condensation phase of chondrogenesis. We will also examine the specific binding of HA to the surface of cells in various phases of differentiation. Finally, the temporal and spatial transitions that occur in the production of cartilage-specific sulfated proteoglycans and the glycoprotein, chondronectin, during the course of limb development and cartilage differentiation will be studied.